IMS/SIP - 183 Progress消息的作用

yyh19820713

关于SIP 183 Progress消息的作用，看到如下一段描述：

The called party's VoLTE UE will return an SDP answer in a SIP 183 Progress message. The SDP answer should contain only one codec and indicates that preconditions are also desired but not yet met at the terminating end and that a confirmation should be sent when QOS preconditions have been met at the originating side and that the media stream is inactive.

- 只能包含一个codec

- 告诉MO端需要做precondition，并且MT端的precondition还没有完成

- MO端QoS precondition达成后需要发送确认消息给MT端

- 目前media stream还处于inactive状态